The present invention is implemented in a distributed data processing system, which is a system consisting of two or more data processing systems which are capable of functioning independently but which are so coupled as to send and receive messages to and from each other.
A local area network (LAN) is an example of a distributed data processing system. A typical LAN comprises a number of autonomous data processing "nodes", each comprising at least a processor and memory. Each node is capable of conducting data processing operations independently. In addition, each node is coupled to a network of other nodes which may be, for example, a loop, star, tree, etc., depending upon the design considerations.
As mentioned above, the present invention finds utility in such a distributed data processing system, since there is a need in such a system for processes which are executing or which are to be executed in the individual nodes to share data and to communicate data among themselves.
A "process", as used herein is a self-contained package of data and executable procedures which operate on that data, comparable to a "task" in other known systems. Within the present invention a process can be thought of as comparable to a set (module) of subroutines in terms of size, complexity, and the way it is used. The difference between processes and subroutines is that processes can be created and terminated dynamically and can execute concurrently with their creator and other sets (modules) of "subroutines".
Within a process the data is totally private and cannot be accessed from the outside (i.e. by other processes). Processes can therefore be used to implement "objects", "modules", or other higher level data abstractions. Each process executes sequentially. Concurrent execution is achieved through multiple processes, possibly executing on multiple processors.
Every process in the distributed data processing system of the present invention has a unique identifier connector by which it can be referenced. The connector is assigned by the system when the process is created. The connector is used by the system to physically locate the process.
Every process also has a non-unique, symbolic "name", which is a variable-length string of characters. In general, the name of a process is known system-wide. To restrict the scope of names, the concept of a "context" is utilized. This concept is described in detail in copending U.S. patent applications having Ser. Nos. 000,621 now U.S. Pat. No. 5,165,018 and 000,624 cited in detail above. Basically, a context is a collection of related processes whose names are not known outside of the context.
A process in one context cannot symbolically communicate with, and does not know about, processes inside other contexts. All interaction across boundaries is by means of messages and pass through a "context process".
A "message" is a buffer containing data which tells a process what to do and/or supplies it with information it needs to carry out its operation. Messages are queued from one process to another by name or connector. Queuing avoids potential synchronization problems and is used instead of semaphores, monitors, etc. The sender of the message is free to continue after the message is sent. When the receiver attempts to get the message, it will be suspended until one arrives if none are already waiting in its queue. Optionally, the sender can specify that it wants to wait for a reply and is suspended until the specific message arrives. Messages from any other source are not dequeued until after that happens.
Messages provide the mechanism by which user transparency is achieved. A process located anywhere in the system may send a message to any other process anywhere within the system if the sending process has the receiving processes name or connector. This permits processes to be dynamically distributed across the system at any time and to gain optimal throughput without changing the processes to which they are referenced. Sending messages by connector obviates the need for a name search and ignores context boundaries. This is the most efficient method of communicating.
In the present invention messages are generally composed of a message ID and one or more "triples". The message ID is a word describing the purpose of the message (e.g. status) or the request (e.g. get) encoded in it. A triple is a data portion made of three fields. The first fields generally identify the type of triple. The second field indicates how many bytes of information are contained in the third field, which may be zero (0). The third field contains the data of the message, such as a process status code.
A problem in a distributed data processing system is the location of password files defining the users' permissions. Since there are generally a number of different nodes in a distributed system, it is desirable to allow a user to logon to the network from any of the nodes and be able to determine the user's permissions. An associated problem is where the system will look to find these password files.
Accordingly, it is an object of the present invention to provide a distributed computer system that overcomes the above deficiencies.
Another object of the present invention is to provide a distributed computer system which will transparently locate and access a password file to determine a user's access permissions.
Still another object of the present invention is to provide a distributed computer system which provides improved configurability of access by definitions at system start-up time.